Entrainment and transport in idealized three-dimensional gravity current simulation

A purely z-coordinate DieCAST ocean model is applied to the DOME idealized bottom density current problem that is patterned after the Denmark Strait. The numerical results show that the background viscosity plays more important role than the chosen coordinate system in the entrainment and mixing if the background viscosity is not small enough. Either higher horizontal viscosity or coarser resolution leads to slower longitudinal propagation. Reducing vertical mixing parameterization also leads to slower longitudinal propagation with larger vertical penetration. The simulation gives consistent results for the moderate and fine resolution runs. At very coarse grid, the dense water descends more slowly and is mainly dominated by diffusion. Time-averaged downstream transport and entrainment are not very sensitive to viscosity after the flow reaches its quasi-steady status. However, more realistic eddies and flow structures are found in low viscosity runs. Larger numerical dissipation is not required/recommended in z-coordinate models. The result shows good convergence of the resolved flow as expected and clarifies the influence of numerical dissipation/mixing on overflow modeling.